1. Field of the Invention
The polymerization of olefins using Ziegler-Natta catalysts is widely utilized. These catalysts provide polyolefins possessing the desired characteristics of these polymers in high yield. However, the use of these conventional catalysts are subject to important failings. Thus, new and improved catalysts are continually being sought. An important class of catalysts where such improvement is sought are those catalysts which aid in the polymerization of the commercially very important alpha-olefin, propylene.
Commonly in the polymerization of many alpha-olefins, especially propylene, a catalyst having a magnesium halide support is utilized. However, when polyolefins, catalytically polymerized with a magnesium halide supported catalyst, are processed into molded products, the molding apparatus processing the polyolefin is subject to corrosion. This corrosion is caused by the residual presence of magnesium halide in the polymeric product. The adverse effect of this corrosion is not limited to damaging expensive molding machinery. More importantly, the polymeric molded article processed in this equipment is often characterized by aesthetic flaws.
Another detrimental property of catalysts, conventionally used in the polymerization of olefins, notably propylene polymers, is caused by their incorporation of internal electron donors. These donors are included in the catalyst to insure that the propylene polymer product is highly isotactic. Those skilled in the art are aware of the criticality of stereoregularity in propylene polymers. However, those skilled in the art are also aware that the presence of internal electron donors creates difficulties. Unless the amount and type of electron donor compound is carefully selected not only is the stereoregularity of the resultant polymer deficient but, in addition, poor catalytic activity often results. This detrimental effect occurs even if the amount and type of electron donor is properly chosen but the catalyst is formed with the electron donor compound added in the wrong sequence.
The utilization of electron donor compounds often creates additional problems involving offensive odors in the final polymeric product. This unfortunate result obtains even if the ideal electron donor compound, in the correct concentration, added at the proper time in the catalyst formation process, is utilized. Thus, polymers polymerized in the presence of catalysts which include an electron donor compound must oftentimes be deashed or deodorized in order to insure the absence of odor in the final product.
Very recently a patent application, a co-applicant of which is co-inventor of the present invention, defined a new catalyst which substantially overcomes the problems discussed above. That is, a new catalyst is therein described which produces olefinic polymers, especially propylene polymers, which possess high stereoregularity, uniform particle size distribution, good spherical morphology and high bulk density. Although this invention represents a significant advance in the art, improvements over it are highly desirable.
Although the catalyst of this new invention provides an activity in excess of those normally obtained in propylene polymerization, it is always desirable to improve this activity. Not only does a higher activity producing catalyst increase the efficiency of the polymerization process but insures a higher purity product. Those skilled in the art are aware that the effect of higher activity not only reduces the amount of catalyst required per unit weight of polymer product but this also translates into lower catalyst concentration in the final polymeric product.
It is also noted that the catalyst of this recent application produces a polymer having excellent bulk density and as a corollary thereof, low fines concentration. However, these properties, like other properties, are always subject to improvement. Those skilled in the art are aware that the greater the bulk density, the greater the productivity of a polymerization process independent of catalyst activity. The greater the bulk density, the greater the weight of polymer produced per unit volume of reactor. The lower the fines concentration, that is, the lower the concentration of very small polymer particles, moreover, the lesser the problem associated with plugging of process equipment, conduits and, especially, filters. Such plugging causes serious interruptions in production schedules.
Another desirable property that the significantly improved catalyst of the recent prior art does not fully address is the catalyst's hydrogen response. Those skilled in the olefin polymerization art are aware that variation of hydrogen concentration in olefin polymerization reactions affects catalyst activity as well as polymer properties. Certain catalysts enhance these results, others diminish them and yet others have little effect.
The above remarks make clear the continuing need in the art for a new olefin polymer catalyst having the desirable properties considered above. They also establish that although recent prior art has significantly addressed these needs further improvements are highly desired in the art.
2. Background of the Prior Art
Japanese Patent Publication 162,607/1983 attempts to eliminate the problem created by halogen-containing carriers. In this disclosure inorganic oxides, such as silica, were proposed as a catalyst support. This carrier, containing no halogen, was reacted with a magnesium dialkoxide and an electron donor, such as a carboxylic acid monoester, and a titanium halide compound.
Even if the allegations made in this disclosure of high catalytic activity, production of a highly stereospecific polymer having a high bulk density and narrow particle size distribution were correct, the problems associated with catalyst odor were not addressed. However, testing of this catalyst established that the catalyst provided less than desired activity and that the olefinic polymer product was wanting in stereoregularity and particle size distribution.
A more recent disclosure, U.S. Pat. No. 4,595,735, provides a catalytic component for the polymerization of olefins prepared by contacting a magnesium alkoxide, a halogenated hydrocarbon, a halogenated silane and a titanium compound. It is emphasized that this catalyst, useful in the polymerization of ethylene homopolymers and copolymers, incorporates a halogenated hydrocarbon. This catalyst is not only principally directed at the polymerization of ethylene polymers but, significantly, emphasizes the formation of high melt index polymers. Those skilled in the art are aware that however useful this catalyst is in ethylenic polymer applications, its application to propylene polymers is restricted. Most propylene polymers are used in applications requiring a polymer of low melt flow rate. That is, the molecular weight of the polymers produced in accordance with the '735 catalyst is significantly lower than that required of polypropylene.
U.S. Pat. No. 4,565,795 sets forth an olefin polymerization catalyst which is prepared by the reaction of a chemically treated silica support with a dihydrocarbyl magnesium compound and a halogenated tetravalent titanium compound. The chemical treatment of the silica support involves the use of a chlorinating compound, an alkanol, a silylating compound, an acid chloride or an organoboron compound. Again, this catalyst includes constituents which are adverse to the production of stereoregular polymers, especially polypropylene. It is thus not surprising that this catalyst is suggested for use in the polymerization of ethylene polymers.
U.S. patent application Ser. No. 326,708, filed Mar. 21, 1989, now U.S. Pat. No. 4,950,631 (Buehler et al.), a co-applicant of which is a co-inventor of the present invention, is the recent application which addresses many but not all of the demands required of olefin catalysts. This catalyst has been discussed earlier.
U.S. Pat. No. 4,394,291 discloses a catalyst useful in the polymerization of olefins. This catalyst involves the reaction of a Group II metal dihalide with a transition metal compound. It is noted that in an alternate embodiment this reaction also involves an electron donor. This product is, in turn, reacted with an organoaluminum compound. Finally, the product of this further reaction is reacted with a halide ion exchanging source. Such a source may be a multiplicity of agents of which the combination of titanium tetrachloride with any one of silicon tetrachloride, trichlorosilane, dichlorophenylsilane and dichlorodiphenylsilane is preferred.
U.S. Pat. No. 4,503,159 describes an olefin polymerization catalyst formed by reacting water with a magnesium dihalide in the presence of a phase transfer catalyst and reacting this product with a benzoic acid ester, an alkoxytitanium compound, an organoaluminum halide and a halogen ion exchanging source. The preferred halogen ion exchanging source is titanium tetrachloride or titanium tetrachloride and a silicon halide which may be trichlorosilane and/or silicon tetrachloride.
U.S. Pat. No. 4,544,716 sets forth a similar catalyst to the '159 patent wherein, again, a halide ion exchanging source is utilized. A particular preferred source is titanium tetrachloride, trichlorosilane and silicon tetrachloride present in a molar ratio in the range of about 2.5:2:1 to 4:3.5:1. The volume of these components are preferably such that the combined volume of the trichlorosilane and silicon tetrachloride equals that of the titanium tetrachloride.
European Patent Application 0 115 833 discusses an olefin polymerization catalyst in which a magnesium dihalide combined with water is reacted with a benzoic acid ester and an alkoxytitanium compound to form a first catalyst component. This first component is reacted with a organoaluminum halide. The solid product of this reaction is reacted with a halide ion exchanging source. The ion exchanging source in a preferred embodiment is titanium tetrachloride, trichlorosilane and silicon tetrachloride.